This invention relates to circuitry for electric appliances and, more specifically, to an improved power shut-off circuit for an electric coffee maker.
In most drip brewing coffee makers, a timing circuit is initially enabled for a specified period of time. During this period, heated water flows into a container holding coffee grounds and, subsequently, brewed coffee drips into a carafe receptacle. A heating element beneath the receptacle keeps the brewed coffee hot during this time period. After the predetermined period of time has expired, power to the heating element is turned off.
Typical examples of such coffee makers with timing circuits for brewing coffee and maintaining the brewed coffee sufficiently hot during the predetermined period of time are found in the following patents.
U.S. Pat. No. 4,330,702 to Cheng discloses an off-the-shelf timer alarm clock integrated circuit implemented as an electric control device for a coffee maker. The timer circuit begins a "keep warm" cycle by initiating a fifty-nine minute drive signal to a semiconductor switching means such as a silicon controlled rectifier or SCR which in turn allows power to flow to a relay. The relay activates a heating element which heats the brewed coffee for the fifty-nine minute period or "keep warm" cycle. If the length of time for this cycle is insufficient, a D.C. power supply latches "on" the SCR, and the relay will remain activated for an indefinite period of time. The only way to turn Cheng's coffee maker off is to do it manually. Accordingly, there is no automatic shut off feature to prevent energizing or re-energizing of the coffee maker; the absence of such a feature is extremely dangerous because an activated or energized unattended coffee maker can cause an electrical malfunction which might lead to a fire.
U.S. Pat. Nos. 4,406,217 to Oota and 4,418,614 to Oota et al are somewhat similar to each other. Both patents disclose a coffee maker including a mill mechanism, a drip mechanism and a control device. Logic circuits of the control device are utilized to select a "regular" or "strong" flavored coffee. There exists a keep warm cycle somewhat similar to that employed in the Cheng patent. However, neither of the Oota patents discloses a positive disconnect means or method at the end of the warm cycle.
A disadvantage with these known timing circuits for electric coffee makers is that a disturbance in the supply line voltage such as a power surge, brown-out or lightning strike, can activate the timer circuitry and turn the appliance "on". This activation happens because these known timing circuits in electric coffee makers are not positively disconnected from the power line source unless the user manually unplugs or disconnects the power cord from the wall socket or power source.
U.S. Pat. No. 4,566,802 to Koehler recognizes a need to prevent energizing or reenergizing of the coffee maker during undesirable periods. In fact, the Koehler patent notes the concern of the Underwriters Laboratories with known coffee makers which repeatedly energize the coffee maker when left unattended. In an attempt to solve this problem of repeatedly activating the coffee maker, the Koehler patent discloses a twenty-four hour clock/controller utilizing an alarm inhibit circuit. The alarm inhibit circuit prevents the clock from recycling after a brewing cycle has initially been set. A cycle enable signal must be activated to re-start the brewing cycle. While no doubt exists as to the objective sought by the Koehler device, the circuitry suffers from a number of weakness which do not successfully prevent the re-energizing or activating of the coffee maker when the coffee maker is left unattended as expressed by the concern of the Underwriters Laboratories. For example, there are circumstances under which the Koehler device can be accidentally reactivated. Typically, because the device is not physically disconnected from the power line source, such as when the user manually unplugs the power cord, a severe lightning storm can cause electrical noise to be superimposed on the power lines and thus scramble the logic circuits of the Koehler device which will reactivate the brewing cycle. With the cycle activated, the coffee maker turns "on". Accordingly, because the Underwriters Laboratories are concerned with fires attributed to activated unattended coffee makers, the need still exists for a device which is de-energized such that any type of power line disturbance will not cause the circuitry in the device to reactivate the appliance.